This invention relates to internal combustion engine systems and, more particularly, to improvements in such systems which decrease noxious emissions, increase operating efficiency, and reduce maintenance requirements. The invention is applicable to combustion turbine power generating systems and other combustion based systems.
The emission of noxious gases by internal combustion engines (including Brayton turbine combustion cycle systems, diesel engines, Otto-cycle spark ignition engines, and the like) is a continuing problem. From an environmental standpoint, the noxious gases are unacceptable pollutants, and government has become heavily involved in the regulation of emission of such noxious gases. The cost to society is very substantial, as measured by the degradation of the environment plus the cost of limiting or controlling such emissions, which is ultimately paid by the consumer. It is also well known that a combustion process which results in a high level of noxious emissions is generally less efficient, from the standpoint of energy output, than the same process operating in a manner which results in a lower level of noxious emissions.
It has long been recognized in the art that controlled injection of water into a combustion engine can, under certain conditions, increase the operating efficiency and reduce the noxious emissions in the exhaust. One drawback of old water injection techniques was the almost immediate corrosion which can occur when water is introduced (either intentionally or during an injection malfunction) directly to hot engine components.
The products of combustion are rich in oxides of nitrogen (NO.sub.x) formed when unburned oxygen combines with nitrogen, the inert component of ambient air at elevated temperatures. NO.sub.x is an environmental hazard. Excess air is technically defined as that quantity in excess of the theoretical quantity required for complete combustion of a fuel. The combustion turbine, to which the present invention has particular although not exclusive application, operates at very high excess air rates. The high excess air rates produce high levels of noxious and toxic NO.sub.x. Fuel-bound nitrogen and sulfur also cause problems.
It is known that NO.sub.x and other noxious emissions can be reduced by water injection in the combustion chamber, which reduces flame temperatures. Unfortunately, heat needed to vaporize the injected water is wasted energy. Also, to preclude scaling, costly ultra-pure water is needed. Externally produced steam has also been used as an injection medium to reduce NO.sub.x, but this requires substantial additional energy and adds additional environmental hazards due to volatile anticorrosive chemicals found in high pressure steam.
In the U.S. Pat. No. 4,313,300, the quantity of NO.sub.x emissions generated by a combined gas turbine-steam boiler power plant is controlled by recycling steam boiler exhaust gas to the air compressor of the gas turbine, thereby increasing the combustor inlet humidity.
In the U.S. Pat. No. 4,231,333 fuel or water injection into an internal combustion engine is controlled based upon detection of predetermined engine conditions, such as pressure within the engine intake manifold.
In the U.S. Pat. No. 4,417,547 atomized fluid from a nozzle is injected into the carburetor of an internal combustion engine, the rate of fluid injection being varied in response to variations in engine speed and engine load.
My U.S. Pat. Nos. 4,667,465, 4,702,074, 4,731,990, 4,773,846, and 4,731,988 disclose internal combustion engine systems which improve over the above described types of systems, and which decrease noxious emissions, increase operating efficiency, and reduce maintenance requirements. These systems disclose, inter alia, that an ultrasonic fog generator can be used to advantage in injecting a fog into the air received by a combustion chamber in order to improve the efficiency of the combustion chamber and/or reduce the noxious emissions in the exhaust of the combustion chamber. A combustion-based system disclosed in my referenced patent is a turbine power generator which includes a source of input air, a source of fuel, a compressor which receives the input air, a combustion chamber which receives air from the output of the compressor and fuel from the source of fuel, a turbine which receives exhaust gases from the combustion chamber, and an electrical generator mechanically coupled with the turbine. A fogging device is provided and communicates with the input air, the fogging device being adapted to receive a fogger air supply and a fogger water supply, and to generate a fog in the source of input air. The fogging device can operate to achieve evaporation to dryness prior to entry into the combustion chamber; the vapor phase of the water being much less harmful than the liquid. A sensor is provided for sensing noxious emissions in the exhaust gases. The fogging device is controlled in accordance with the sensed noxious emissions. My referenced patents also disclose that heat exchange from the combustion exhaust can be used in a controlled manner to heat the input air to be fogged, that the fog can be controlled by a humidity and dewpoint sensor to assure only a vapor phase at entry to the combustion chamber, and that a supply of chemical suitable for reacting with a component of the noxious emissions (for example calcium carbonate or bicarbonate or calcium oxide to react with sulfur in noxious sulfur dioxide) can be combined with the water supplied to the fogging device.
My referenced patents describe a number of advantages of the disclosed techniques, including the following:
The formation of NO.sub.x is endothermic and removes available energy. Accordingly, by reducing NO.sub.x formation, a substantial increase in efficiency can be obtained.
Excess air is diminished as fog vapor displaces excess air at the input to the combustion chamber. Less excess air can result in less NO.sub.x formation.
When a fog, which has droplets of about 10 microns or less size, is produced, minerals dissolved therein are evaporated to dryness and travel, with little deleterious effect, through engine components as sub-micron dust particles. This can reduce the need for expensive ultrapure water, often used for engine water injection in industry.
The use of an ultrasonic fogger is advantageous in providing a uniform humidity in the air which enters the combustion chamber, which overcomes uniformity problems associated with some water injection techniques.
By modifying the fog water content as a function of the measured noxious emissions, the fog can be precisely tailored to minimize noxious emissions.
Although the described type of combustion-based system with fog injection provides substantial improvements over previous approaches, there is room for further improvement. In the humidification process the zone between saturation and super-saturation has a fringe area due to droplet accumulation created by dynamic impact and agglomeration of droplets and this zone is to be avoided for an engine air intake. A safe humidifier operating range is found at about 80% of saturation where droplets resulting from any intake air humidification may be absorbed because of statistical molecular dynamic relationships between air (gaseous molecules) and small droplets (capillary bound bundles of water molecules) in collisions that permit liquid absorption. This means that actual humidifier output must be limited to something under the rate of water required to saturate the air stream because no humidifier can saturate, with precise uniformity, the air stream being humidified. This also means that the air is superheated with respect to its dewpoint temperature. The fraction of the air that is not saturated is overheated. Overheated air can result in loss of capacity and the tendency to increase noxious emissions in the exhaust gases. Accordingly, operating within a controlled small optimum temperature range, while providing the desired moisture in the input air for reduction of noxious emissions, can be problematic. It is among the objects of the present invention to address this problem.
Compressor and turbine blade fouling is a problem which also substantially impacts engine performance and noxious emission in power plant operations. Prior art solutions have generally been inadequate. Reference can be made, for example, to Thames et al., "On Line Compressor Washing Practices And Benefits", pp. 1-6 (1989), which shows percentage losses attributable to blade fouling and advocates on-line crank-wash equipment with detergents to minimize crank-wash shutdowns. Thames et al. also presents the pros and cons of aspirated talcs for blade polishing as compared to crankwash cycles. With either case, falling efficiencies caused by compressor blade fouling will cause power output to fall unless additional fuel is added to hold engine RPM constant. Additional fuel use, at lowered engine efficiency results in increased combustion temperatures which in turn raise NO.sub.x emissions. It is also among the objects of the invention to address the problem of compressor and turbine blade fouling in prior art power generation systems.
As noted above, heat exchange from exhaust gases has been used in prior art systems for functions such as preheating of input air. It is among the further objects of the invention to address further problem of wasted heat from power generation equipment and achieve additional efficiencies.
Further features and advantages of the invention will become more readily apparent from the following detailed description and taken in conjunction with the accompanying drawings.